The addition of relatively small amounts of a corrosion inhibitor to a system retards or prevents corrosive degradation of the metal. Because corrosion inhibitors are typically added to a system in relatively small dosages, for example, less than 10 percent by weight of total fluid in the system, the expense associated with the additive program is typically justified by reduced maintenance costs and longer equipment life. A low-cost corrosion inhibitor would be particularly beneficial, not only from the standpoint of operating costs, but also because its low cost would effectively remove any economic constraints on the maximum allowed dosage.
Acid gas sorption processes, commonly used in the oil refining, natural gas recovery, and wood pulp industries, generally require alloy construction and/or the addition of corrosion inhibitors to prolong the life of the process equipment. These process units remove H.sub.2 S and CO.sub.2 from gaseous process streams, typically by countercurrently contacting an aqueous solution containing from about 20% to about 50% by weight of an alkanolamine with a gas stream containing H.sub.2 S and/or CO.sub.2. As used herein, the terms "alkanolamine" and "ethanolamine" are generic terms including, but not limited to, monoethanolamine, diethanolamine, triethanolamine, and methyl diethanolamine.
The removal of hydrogen sulfide from gaseous streams, such as the waste gases liberated in the course of various chemical and industrial processes, for example, in wood pulping, natural gas and crude oil production and in petroleum refining, has become increasingly important in combating atmospheric pollution. Hydrogen sulfide containing gases not only have an offensive odor, but such gases may cause damage to vegetation, painted surfaces and wildlife, and further may constitute a significant health hazard to humans. Government-wide regulations have increasingly imposed lower tolerances on the content of hydrogen sulfide which can be vented to the atmosphere, and it is now imperative in many localities to remove virtually all the hydrogen sulfide under the penalty of an absolute ban on continuing operation of a plant or the like which produces the hydrogen sulfide-containing gaseous stream. Solutions of water and one or more the alkanolamines are widely used in industry to remove hydrogen sulfide and carbon dioxide from such gaseous streams.
Corrosion in alkanolamine units significantly increases both operating and maintenance costs. The mechanisms of corrosive attack include general corrosive thinning, corrosion-erosion, and stress-corrosion cracking. Corrosion control techniques include the use of more expensive corrosion and erosion resistant alloys, continuous or periodic removal of corrosion-promoting agents in suspended solids by filtration, activated carbon adsorption, addition of corrosion inhibitors, or purging of the circulating alkanolamine. See, for example, Kohl, A. L. and Reisenfeld, F. C., Gas Purification, Gulf Publishing Company, Houston, 1979, pp. 91-105, as well as K. F. Butwell, D. J. Kubec and P. W. Sigmund, "Alkanolamine Treating", Hydrocarbon Processing, March, 1982.
U.S. Pat. No. 4,795,565 to Yan describes a process for removing heat stable salts from an ethanolamine system by the use of ion exchange resins. The disclosure of U.S. Pat. No. 4,795,565 to Yan is incorporated herein by reference for the operating details both of an ethanolamine acid gas sorption system as well as for the heat stable salt removal process. See also Keller et al., Heat Stable Salt Removal From Amines by the HSSX Process Using Ion Exchange, presented to The Laurence Reid Gas Conditioning Conference, March 2, 1992.
The chemistry of alkanolamine degradation is discussed in the Butwell et al. article cited above. Briefly, the Butwell et al. article notes that monoethanolamine (MEA) irreversibly degrades to N-(2-hydroxyethyl) ethylene diamine (HEED). HEED shows reduced acid gas removal properties and becomes corrosive at concentrations of at least about 0.4% by weight.
Diglycolamine (DGA), on the other hand, is said to produce a degradation product upon reaction with CO.sub.2 which exhibits different properties. DGA is a registered trademark of Texaco, Inc. which identifies an amine having the chemical formula NH.sub.2 --C.sub.2 H.sub.4 --O-C.sub.2 H.sub.4 -OH. DGA degrades in the presence of CO.sub.2 to form N,N'-bis(hydroxyethoxyethyl) urea (BHEEU) which is similar to HEED in corrosivity but differs in that BHEEU has no acid gas removal properties.
Diethanolamine (DEA) reacts with CO.sub.2 to form N,N'-di(2-hydroxyethyl) piperazine. Unlike HEED and BHEEU, the piperazine compound is noncorrosive and has acid gas removal properties essentially equal to its parent, DEA. See the Butwell et al. article at page 113.
Diisopropylamine (DIPA) readily degrades in the contact with CO.sub.2 to form 3-(2-hydroxypropyl) 5-methyl oxazolidone which shows essentially no acid gas removal properties. See the Butwell et al. article at page 113.
U.S. Pat. No. 4,281,200 to Snoble teaches a process for recovering diisopropanolamine from the cyclic reaction products formed by reacting CO.sub.2 with diisopropanolamine which process comprises reacting the cyclic product with an inorganic base at temperatures between about 105.degree. and 200.degree. C.
U.S. Pat. No. 4,971,718 to McCullough et al. teaches a method for treating a gas stream with an aqueous solution containing an monoethanolamine, a methyldiethanolamine, and antimony in concentration of at least 100 ppm.
U.S. Pat. No. 4,944,917 to Madden et al. discloses a method for inhibiting corrosion in an aqueous amine scrubbing solution in contact with H.sub.2 S, which method contacting the aqueous amine solution with H.sub.2 S in the presence of an ammonium or alkali-metal thiosulfate salt and an effective amount of sulfide and/or hydrosulfide ions.
U.S. Pat. No. 4,857,283 to Madden teaches the use of sulfur dioxide for inhibiting corrosion in an amine-containing acid gas scrubbing solution.
U.S. Pat. No. 4,764,354 to Kubek et al. discloses a method for reducing the corrosion rate of carbon steel in contact with an alkanolamine solution in an acid gas scrubbing process, which method comprises maintaining specified levels of hydrogen sulfide and vanadium in the plus five valence state in the alkanolamine solution.
U.S. Pat. No. 4,690,740 to Cringle et al. relates to a technique for inhibiting corrosion in an alkanolamine acid gas sorption system which comprises maintaining a copper ion in the plus two oxidation state by applying an induced or impressed voltage across a point, or across several points in the circulating copper-containing solution.
U.S. Pat. No. 4,631,138 relates to the use of triazones and triazine thiones as corrosion inhibitors.
U.S. Pat. No. 4,596,849 to Henson et al. teaches a corrosion inhibiting composition for ferrous metals and alloys, which composition includes a thiourea-amine-formaldehyde based polymer, and, preferably, a cupric ion-producing material. U.S. Pat. No. 4,595,723 to Henson et al. teaches an additive having a composition similar to that disclosed in the '849 Henson et al. patent, but preferably contains a nickel ion-producing material.
U.S. Pat. No. 4,590,036 relates to an additive for inhibiting corrosion in an amine-containing gas scrubbing system comprising a mixture of antimony and molybdenum metal salts.
U.S. Pat. No. 4,502,979 discloses corrosion inhibiting compositions for use in alkanolamine solutions comprising combinations of vanadium compounds and an organic compound selected from the group consisting of nitro-substituted aromatic acids, nitro-substituted acid salts, and 1,4-naphthoquinone, preferably from the group consisting of p-nitrobenzoic acid, m-nitrobenzoic acid, 3,5-dinitrobenzoic acid, p-nitrophenol, m-nitrophenol, m-nitrobenesulfonic acid, 1,4-naphthoquinone, and mixtures thereof.
U.S. Pat. No. 4,499,003 relates to an aqueous corrosion inhibitor composition comprising soluble antimony and molybdenum salts wherein the weight ratio of soluble antimony salt to molybdenum salt ranges between 0.01 and 1 and about 5 to 1.